1. Field of the Invention
The present invention relates to a rotational angle detecting device for detecting a rotational angle of a rotating member, a torque detecting device for detecting a rotational torque applied to a rotating shaft, and an automobile steering apparatus provided with the rotational angle detecting device and the torque detecting device.
2. Description of Related Art
An electric power steering apparatus which is constructed to drive an electric motor for assisting steering on the basis of a detection result of a steering torque applied to a steering wheel for steering to transmit a rotating force of the electric motor to a steering mechanism for assisting steering has an advantage in that the assisting power characteristics can be easily controlled in accordance with the running state such as high and low speed of the automobile, the frequency of steering, and the like as compared with a hydraulic pressure power steering apparatus using a hydraulic pressure actuator as a generation source of a steering assisting power. As a consequence, the scope of application of an electric power steering apparatus has tended to expand in recent years.
In such an electric power steering apparatus, a steering torque is detected at an intermediate portion of a steering shaft for connecting the steering wheel and a steering mechanism by detecting the rotational angle at two positions separated in the shaft length direction. That is, on the steering shaft which is operated for steering, torsion is generated with an action of the steering torque which is applied to the steering wheel. This torsion corresponds to a difference in the detection angle at two positions described above, so that the rotational torque (steering torque) applied to the steering shaft can be determined by using this difference.
Furthermore, in actuality, the steering shaft is constructed in such a manner that a first shaft (an input shaft) on the steering wheel side and a second shaft (an output shaft) on the steering apparatus side are separated, and these two shafts are coaxially connected via a torsion bar having a small diameter as a torsion element, so that a large relative angle deviation is generated with the torsion of a torsion bar between the first shaft and the second shaft at the time of action of the above steering torque. Consequently, the torque is detected with a high degree of accuracy on the basis of a difference in the detection angle.
Furthermore, in the electric power steering apparatus, it is necessary to detect the rotational angle of the steering shaft, namely the steering angle from the mid-point of the steering angle in order to use the drive control of the electric motor for assisting the steering. In the torque detecting device described above, the steering angle can be detected by the rotational angle detected at two positions of the steering shaft.
The applicant of the present invention proposed in Japanese Patent Application Laid-Open No. 2002-107112 a detecting device which can be preferably used in the detection of the rotational angle and the rotational torque as described above. In the schematic view of FIG. 1, a construction of the rotational angle and torque detecting device is shown.
These conventional rotational angle and torque detecting device, as shown in FIG. 1, is provided with target plates (rotating members) 30 and 40 both having a disc-like shape which are externally fitted onto and fixed to both the first shaft 1 and the second shaft 2 in the vicinity of a connection portion of the first shaft 1 and the second shaft 2 coaxially connected via a torsion bar T. MR (magnetic resistance effect) sensors 50 and 60 as detecting means are fixed and arranged so as to be positioned opposite to the outer circumferencial surface of these target plates 30 and 40.
On the outer circumferential surface of the target plates 30 and 40 where MR sensors 50 and 60 are positioned opposite to each other, targets 30a and 40a are provided as detection targets. These targets 30a and 40a are, as shown in FIG. 1, protrusions made of a magnetic member constructed as a partially helical continuous member whose inclination direction with respect to an axial direction is cyclically changed, and is present as a magnetically discontinuous portion with respect to the peripheral portion. Such targets 30a and 40a reciprocate in an axial direction in accordance with the rotation of the target plates 30 and 40 when viewing the fixed position of the MR sensors 50 and 60 as a reference. Then, the MR sensors 50 and 60 are constructed so as to generate a sine wave-like output which changes cyclically in accordance with the axial direction deviation of the targets 30a and 40a. 
FIG. 2 is a view for explaining an operation of detecting rotational angle with the conventional rotational angle and torque detecting device described above. As shown in FIG. 2, the MR sensor 50 is constructed in such a manner that a pair of MR (magnetic resistance effect) elements 50a and 50b having a characteristic of changing the electric resistance with the action of the magnetic field are connected in series, and an internal magnet 50c is arranged so as to allow an equally magnetic field to act on one side of the pair of MR elements 50a and 50b. With such a construction, an electric potential, when a predetermined voltage is applied to a series circuit of the MR elements 50a and 50b, can be taken out as an output.
FIG. 2 is a view showing a state in which the target 30a provided on an outer circumference of the target plate 30 is developed on a plane. The MR sensor 50 which is constructed in a manner as described above is arranged, so that the other side (the opposite side of internal magnet 50c) of MR elements 50a and 50b is positioned approximately at the center of the deviation area of the above-described target 30a. When the target plate 30 is rotated in this state, the MR sensor 50 relatively changes its position in a circumferential direction along the outer circumference of the target plate 30 and the target 30a on the outer circumference of the target plate 30 is deviated in the direction of a parallel arrangement of MR elements 50a and 50b. 
Here, since the target 30a is made of a magnetic member, the magnetic field formed by the internal magnet 50c is deviated to either one of the MR elements 50a and 50b in accordance with a deviation of the target 30a. That is, in the case where the MR sensor 50 is positioned at point A in FIG. 2, the magnetic field formed by the internal magnet 50c is deviated to the MR element 50a side which relatively comes close to the target 30a, so that the resistance of the MR element 50a becomes larger while the resistance of the other MR element 50b becomes small. Consequently, the output taken out between both MR elements 50a and 50b becomes minimum. On the other hand, in the case where the MR sensor 50 is positioned at point B in FIG. 2, the magnetic field formed by the internal magnet 50c is deviated to the MR element 50b side which comes relatively close to the target 30a so that the resistance of the MR element 50b increases while the resistance of the other MR element 50a decreases, so that the output taken between both MR elements 50a and 50b becomes maximum.
In this manner, the output of the MR sensor 50 becomes an output having a sine wave-like form which cyclically changes between the minimum output at the point A and the maximum output at the point B in accordance with the target 30a caused by the rotation of the target plate 30. Consequently, this output is taken in at a predetermined sampling cycle to calculate an accumulated value of output change from the previous sampling time, so that the rotational angle of the target plate 30 and the first shaft 1 are obtained.
Also, at the MR sensor 60 arranged opposite to the target plate 40 on the second shaft 2 side, completely the same output can be obtained in response to the target 40a on the outer circumference of the target plate 40. On the basis of the output change, the rotational angle of the target plate 40 and the second shaft 2 can be obtained. Furthermore, with a difference of the rotational angle between the first shaft 1 and the second shaft 2 which are obtained in such manner, a rotational torque applied to both shafts 1 and 2 can be determined.
However, in the conventional rotational angle and torque detecting device which are configured in the described manner, there is a problem in that when the MR sensors 50 and 60 which are fixedly arranged and the target plates 30 and 40 which rotate integrally with the first shaft 1 and the second shaft 2 are assembled at a deviated position in an axial direction, such a positional deviation affects the output of the MR sensors 50 and 60, so that the rotational angle and the torque accuracy which are obtained with this output change are lowered.
FIG. 3A and FIG. 3B are views for explaining problems in the rotational angle and torque detecting device as described above. FIG. 3A is a view showing a state in which the MR sensor 50 is deviated in position upwardly with respect to the target plate 30. FIG. 3B is a view showing a state in which the MR sensor 50 is deviated in position downwardly in the same manner. Respectively, FIG. 3A and FIG. 3B, the output waveforms obtained in the respective cases are shown together as well.
In the state shown in FIG. 3A, since the detection area of the MR sensor 50 is deviated upwardly in the formation area of the target 30a provided on the target plate 30, the detection area of the MR sensor 50 comes to exclude an inclination changing portion on the lower side of the target 30a. As a consequence, the output waveform of the MR sensor 50 is different from those on upper and lower half portions. As shown in FIG. 3A, the lower half portion of the output corresponding to the upper half portion of the target 30a becomes a waveform having a gentle peak value whereas the upper half portion of the output corresponding to the lower half portion of the target 30a becomes a waveform having a sharp peak value.
On the other hand, in the state shown in FIG. 3B, since the detection area of the MR sensor 50 is deviated downwardly in the formation area of the target 30a, the detection area of the MR sensor 50 comes to exclude the inclination changing portion on the upper side of the target 30a. As a consequence, the output waveform of the MR sensor 50 comes to have an upper half portion with a gentle peak value and a lower half portion with a sharp peak value.
In a case where the output of such the MR sensor 50 is obtained, an error is generated in the rotational angle of the target plate 30 and the first shaft 1 obtained by accumulating a variation quantity of the output of the MR sensor 50. Completely in the same manner, an error is also generated in the rotational angle of the target plate 40 and the second shaft 2 obtained on the basis of the MR Sensor 60. Furthermore, an accuracy of the rotational torque obtained from the rotational angle difference between the first shaft 1 and the second shaft 2 is also lowered as well.
Furthermore, in a case where the target plates 30 and 40 are attached inclined with respect to the first shaft 1 and the second shaft 2, or in a case where the first shaft 1 and the second shaft 2 to which the target plates 30 and 40 are attached are twisted and turned, directions of the rotational deviation of the MR sensors 50 and 60 change according to the rotation of respective targets 30 and 40. Consequently, the output waveform shown in FIG. 3A and FIG. 3B exist simultaneously in the output obtained in one rotation of the target plates 30 and 40, so that the detection accuracy of the rotational angle and the rotational torque is further lowered.
Therefore, conventionally, in order to discard error factors of the output of the MR sensors 50 and 60 resulting from the positional deviation in the axial direction of the target plates 30 and 40 described above, it is required to assemble MR sensors 50 and 60 and the target plates 30 and 40 with a high accuracy. Furthermore, generation of twist and turn is prevented by increasing the support rigidity of the target plates 30 and 40. Thus, there is a problem in that a large amount of labor and time is required for countermeasures with the above problem.
The present invention has been made in view of the above circumstances. An object of the invention is to provide a rotational angle detecting device and a torque detecting device which enable detection with a high accuracy by effectively preventing the generation of a detection error of the rotational angle and the rotational torque resulting from a positional deviation in the axial direction of the rotating member and the detecting means without requiring labor and time at the time of assembling. Furthermore, an object of the invention is to provide a steering apparatus using these devices for automobiles.
A rotational angle detecting device according to a first aspect of the present invention comprising: a rotating member; a target provided on a rotational circumference of the rotating member; and detecting means for cyclically changing an output in accordance with a deviation of the target with the rotation of the rotating member; whereby a rotational angle of the rotating member is detected on the basis of the output change of the detecting means; characterized in that the target is a magnetically discontinuous portion which is parallelly provided in a circumferential direction of the rotating member; and the detecting means is constructed in such a manner that a pair of detection elements, which sense magnetism, are arranged in a circumferential direction in accordance with the arrangement cycle of the target to enable obtaining an output between both the detection elements.
In this first aspect of the present invention, magnetically discontinuous portions are parallelly provided in a circumferential direction on a rotational circumference of the rotating member to form targets and a deviation of the targets in the circumferential direction with the rotation of the rotating member being taken out as an output between a pair of detection elements which sense magnetism and arranged in a circumferential direction so as to correspond to a parallel arrangement interval of the targets, and a rotational angle is detected on the basis of a change in this output.
Furthermore, the rotational angle detecting device according to a second aspect of the present invention is characterized in that the pair of detection elements of the first aspect are magnetic resistance effect elements which change electric resistances with action of the magnetic field.
In this second aspect of the present invention, an MR element (magnetic resistance effect element) which changes electric resistance by action of the magnetic field is used as a detection element and the fact that an electric resistance of a pair of MR elements arranged in a circumferential direction of the rotating member is changed in accordance with a deviation of the targets with rotation of the rotating member so as to easily obtain the rotational angle of the rotating member on the basis of a change in the output voltage of a bridge circuit including both MR elements.
Furthermore, the rotational angle detecting device according to a third aspect of the present invention is characterized in that the rotating member of the first or second aspect is a gear provided with a plurality of teeth made of a magnetic member equally arranged on the outer circumferential surface as the target. Furthermore, the rotational angle detecting device according to a fourth aspect of the present invention is characterized in that the rotating member of the first or second aspect is a ring made of a magnetic member being provided with a plurality of slits equally arranged on the outer circumferential surface as the target. Furthermore, the rotational angle detecting device according to a fifth aspect of the present invention is characterized in that the rotating member of the first or second aspect is a disk being provided with magnetic poles having different polarities which are equally and alternately arranged on the outer circumferential surface as the target.
In the third through the fifth aspects of the present invention, a rotating member which is formed by a parallelly provided targets in a circumferential direction as a magnetically discontinuous portion is simply composed of a gear provided with teeth made of a magnetic member, a ring made of a magnetic member having a slit formed on a circumferential surface thereof, and a disk which is alternately magnetized to both N and S poles.
Furthermore, the torque detecting device according to a sixth aspect of the present invention is a torque detecting device for detecting a rotational torque applied to a first shaft and a second shaft coaxially connected via a torsion element by a relative angle deviation between the first shaft and the second shaft caused by the torsion of the torsion element, wherein: the rotational angle detecting device any one of the first through fifth aspects attached respectively on the first shaft and the second shaft; and torque calculating means for calculating the rotational torque on the basis of the rotational angle difference detected respectively by the rotational angle detecting devices.
In this sixth aspect of the present invention, a rotational angle detecting device is arranged which enables detecting an accurate angle without being affected by the axial direction position of the detecting means respectively on the first and the second shaft connected via the torsion elements, so that a rotational torque is detected with a high accuracy which is applied to the first and the second shaft based on the difference in the rotational angle detected by these rotational angle detecting devices.
Furthermore, a steering apparatus according to a seventh aspect of the present invention is characterized by comprising: the rotational angle detecting devices any one of the first through fifth aspects and/or the torque detecting device of the sixth aspect at the middle of the steering shaft for connecting the steering wheel and the steering mechanism.
In this seventh aspect of the present invention, the rotational angle detecting device and the torque detecting device as described above applied to the steering apparatus of automobiles to obtain an accurate detection value of the steering angle and the steering torque, so that the result is used for various types of control required for a high reliability such as a drive control or the like of an electric motor for steering assistance in the electric power steering apparatus.
The above and further object and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.